The present invention relates to an improvement in a method for increasing drive torque with wheel speeds being detected at least on the driven wheels, a differential being arranged on each driven axle to compensate for a speed difference between wheels on one vehicle side and wheels on the other vehicle side of this axle, a braking torque being produced, when a cut-in threshold value for the speed difference is exceeded, on the wheels on one vehicle side of the driven axle which have a higher wheel speed, and the braking torque being regulated as a function of the speed difference.
The periodical Auto Motor Sport, 16/86 of Feb. 8, 1986, pages 34ff, describes the action of a differential lock in that controlled brake engagement takes place on one side of a driven axle provided that there is a speed difference between the wheels on the different sides of a driven vehicle axle. Only the speed difference between the wheels on the left and right vehicle sides of a driven axle is taken into account. It is not possible to compare the speeds with each other and derive different driving conditions. The triggering threshold must, for example in order to exhibit cornering tolerance, allow a large speed difference before the regulated brake engagement is implemented.
An object of the present invention is to improve a drive-torque-increasing method in that the triggering threshold decreases, and therefore the regulation engagement can take place, even when the speed differences between the wheels on the two sides of the driven axle are small.
This object has been achieved according to the method of the present invention by subjecting the determined wheel speeds of the wheels of driven axles to a correction on the basis of a wheel calibration and to a correction on the basis of cornering detection, determining the speed difference of the wheels of a driven axle on the basis of values of the wheel speeds obtained by the wheel calibration and the cornering correction, and determining a reference velocity representing the velocity of the vehicle, with the braking torque being produced only when the wheel velocities corresponding to the wheel speeds are larger than the reference velocity on both sides of the driven axle.
On the basis of the fact that a reference velocity representing the vehicle velocity is determined, a value for the slip of the wheel, for which a measured value of the wheel speed exists, can be calculated. A calibration of the wheel speeds can as a result also then take place. By virtue of the calibration of the wheel speeds, the differences in the wheel speed which arise on the basis of a mutual difference in the rolling circumference of the wheels can be computer-compensated. A wheel calibration which can be applied in the case of the method according to the present invention is described in unpublished DE 43 27 491 A1 (see corresponding above cross-referenced U.S. patent application "PROCEDURE FOR CALIBRATING THE WHEEL SPEEDS FOR A MOTOR VEHICLE" which is incorporated by reference herein) of applicants' assignee and in this respect explicit reference is made thereto.
Cornering correction takes place in a further step. The speed difference of the wheels on the basis of the different cornering radius of the wheels on the two vehicle sides is determined from the reference velocity and from the wheel speeds as well as from the geometrical conditions of the vehicle. It can, for example, further be assumed that the vehicle fulfills the Ackermann conditions during cornering. This assumption then gives the ratios of the wheel speeds as a function of the geometrical conditions of the vehicle.
After the wheel calibration and the cornering correction for the wheels on both sides of a driven axle have been completed, the speed difference is determined therefrom. The speed difference thus determined is substantially based on the difference in the adhesion on the two sides of this axle. It is therefore possible to feed brake pressure into the wheel brake cylinder and therefore to produce a braking torque on the side at which the larger wheel speed is established even when the speed difference is small. The braking torque thus produced produces the same effect as the partial blocking of the differential.
The wheel speeds of non-driven wheels and the wheel speeds of driven wheels are generally used for forming the reference velocity. Methods for determining a reference velocity are known in various ways, for example in the case of anti-lock systems. A method for determining a reference velocity and for determining the cornering-dependent wheel speeds is described in above cross-referenced U.S. patent application "METHOD FOR CONTROLLING VEHICLE BRAKE PRESSURE AS A FUNCTION OF THE DEVIATION OF THE ACTUAL SLIP OF WHEELS RELATIVE TO A DESIRED SLIP", incorporated by reference herein of applicants' assignee and in this respect explicit reference is made thereto.
According to a further aspect of the invention, the engine speed is advantageously taken into account when determining the braking torque to be produced. If the engine speed data is not available, then sufficiently accurately conclusions regarding the engine speed can be drawn with the aid of the average driving-axle velocity. In this situation, the first gear of the manual gearbox is assumed to be selected with the effect that stalling of the engine due to the regulation engagement is avoided even when the driver requires only low engine power, contrary to what is still the case in known systems.
Thus, brake-pressure production is, on one hand, suppressed at low engine speeds or at low driving-axle velocity, and, on the other hand, a limited speed difference between the wheels on the two sides of a driven axle can intentionally be tolerated. This results from an auxiliary value which is added to the cut-in threshold, ES, being determined as a function of the engine speed and of the brake pressure produced. The cut-in threshold increased by the auxiliary value then forms the basis of the brake-pressure regulation.